Drum mix asphalt plants for use in preparing asphalt aggregate paving compositions are well known in the art. A typical drum mix asphalt plant is disclosed in U.S. Pat. No. 4,211,490. The plant includes a drum mixer having a drying zone wherein virgin aggregate is dried by agitating the aggregate in a flow of heated air; and a mixing zone wherein the aggregate material is mixed with liquid asphalt to form the desired mixture. The exhaust from the drum is then drawn through a baghouse, where fiber filter elements remove aggregate dust from the airflow. Aggregate dust thus recovered can be re-admitted into the drum to be coated with liquid asphalt and become part of the asphalt composition.
The heat for the drying zone of such drum mixers is typically provided by a burner mounted in the upper end of the drum. Early asphalt plants were fired using coal-fired boilers with a portion of the exhaust from the coal fire going to dry the aggregate. However, crude oil and natural gas soon replaced coal as the fuels of choice in asphalt plants, being much easier to handle and cleaner burning. Hence, nearly all asphalt plants in recent history have used fuel oil or natural gas fired burners to heat and dry the aggregate.
Over the years, natural gas and fuel oil have continued to escalate in price so that the cost per million BTU of most liquid and gaseous fuels is from three to five times that of the solid fuels. Accordingly, a significant savings could be achieved by utilizing coal to fire the asphalt plant. However, since coal tends to burn much dirtier than fuel oil or natural gas, there remained the problem of providing a coal-fired burner which meets pollution standards.
Additionally, pulverized coal creates storage problems, inasmuch as the coal dust is potentially explosive. Thus, it has been considered necessary to pulverize the coal only as needed by the burner to prevent the dangerous storage of pulverized coal. A number of efforts have been made to provide a simple direct fired system that will feed the coal directly into a pulverizer, and then send it out of the pulverizer directly into the burner without any intermediate storage of the pulverized coal particles. However, a significant time lapse has occurred between the time the perception of a need for additional coal and the time the coal is fed into the pulverizer, processed, and supplied to the burner. Therefore, changes of production rate have resulted in very sluggish control. Accordingly, there is a need to provide a coal-fired asphalt plant which allows instantaneous control of the burner without the storage of large quantities of pulverized coal.
Typically, coal for burning in an asphalt plant has been pulverized without specific regard to classification of the particle size. As a result, typically 70-80% of the feed is reduced to -200 mesh. The combustion of coal particles larger than 200 mesh results in an extremely long flame, an undesirable characteristic for a heat source of a drum mix asphalt plant. A long flame increases the temperature at the mixing zone of the drum where liquid asphalt is introduced. Liquid asphalt exposed to high temperatures and steam in the drum mixer produces vapors comprising light end hydrocarbons which are stripped from the liquid asphalt. When a baghouse is used to treat exhaust gases, these light end hydrocarbons appear as oil buildup on the filter elements of the baghouse and are also released through the stack, creating air pollution problems. Many light ends which remain as vapor through the baghouse condense after being exposed to low temperature air on discharge from the plant, and in extreme cases can result in oil stains forming on objects in areas around the asphalt plant. When the plant is operated with this type of process for a sufficient time, there is a high probability of fire occurring in the baghouse, because a spark from burning materials in the drum can ignite oil-soaked bags and damage or destroy the entire baghouse. In addition, the oil which forms in the baghouse can combine with dust to clog the filter elements so that air can no longer pass through, reducing plant productivity and creating difficult cleaning problems. Accordingly, it is necessary to confine the heat to the drying zone of the drum, so as not to expose the liquid asphalt in the mixing zone to high temperatures, an object which is inconsistent with the long flame created by the burning of large coal particles.
In order not to have the coal-fired burner sending long flames down the drum into the mixing zone, it has been necessary to provide a separate combustion chamber within which the coal-fired burner is disposed. However, such combustion chambers require frequent and expensive maintenance. The refractory of the combustion chamber is subject to frequent expansion and contraction as a result of the heating and cooling when the burner is started and stopped. Over a short period of time, uneven expansion can cause structural failure of the refractory, requiring the asphalt plant to be shut down while the refractory is repaired or replaced. Coal slag also can adhere to the refractory surface, subsequently expanding and contracting at a different rate than the refractory material, and result in the erosion of the refractory surface. Accordingly, there is a need to provide a coal-fired asphalt plant which does not require a separate combustion chamber for the burner.
In typical coal-fired asphalt plants of the prior art, large particles are thrown into the burner and are slow to burn. They can be carried down the dryer and all the way to the baghouse before they are totally consumed, creating a condition that can lead to baghouse fires and other serious problems. Thus, there is a need to provide a coal-fired asphalt plant which prevents the possibility of oversized coal particles entering the burner and being carried throughout the plant.
Another inherent problem with conventional coal-fired burners is the emission of sulfur dioxide, which can cause acid-rain. To prevent excess sulfur dioxide emissions, conventional coal-fired burners require expensive fluegas desulfurization equipment. Accordingly, there is a need to provide a coal-fired asphalt plant which emits acceptable levels of sulfur dioxide without the need for expensive fluegas desulfurization equipment.